This invention relates to a method and apparatus for removing vascular obstructions, such as atheromatous plaque and thrombi. More particularly, the invention relates to the use of a laser, preferably an excimer laser, for performing surgically smooth excisions of eccentric obstructions and containing the excised section.
Several techniques exist for enlarging an obstructed arterial lumen. The most common procedure is known as balloon angioplasty. Using this technique, the artery is dilated by inflating a balloon catheter within the lumen of the artery. The angioplasty balloon reduces arterial obstruction by eccentrically displacing atheromatous lesions through vessel wall dissection. The inadequacies of balloon dilation relate to the fact that the technique fractures, but does not physically remove, atheromatous plaque. The resulting roughened arterial surface causes the formation of blood clots (thrombi). In addition, the continued presence of the atheromatous material in the artery acts as a nucleus for the redeposition of additional atheromatous material (restenosis). Approximately 20-30% of arteries treated with balloon dilation experience restenosis within 6 months.
Another device for displacing vascular obstructions, called a "cut-and-retrieve" system, is described in U.S. Pat. No. 4,669,469, issued June 2, 1987, for a "Single Lumen Atherectomy Catheter Device". The balloon inflatable device uses a fixed guidewire attached at its tip for positioning within the blood vessel. Properly positioned, the device causes prolapse of an eccentric vascular obstruction into the cavity of a metal housing attached to a vascular catheter. The metal housing is essentially an enlarged cylindrical cavity having a cut-out window and a cup-shaped blade positioned just below this window. A balloon is located opposite from the window such that inflating the balloon pushes the window against the arterial wall causing prolapse of the eccentric obstruction into the cavity of the metal housing. A section of the obstruction is then excised by the rotating blade as it is advanced along the length of the window. The excised section is contained within the housing and is physically removed from the patient's body. By physically removing the atheromatous material and leaving a surgically smooth arterial wall surface, the incidence of restenosis is reduced. Unfortunately, the mechanical requirements of a housing able to contain a high speed (about 2,000 r.p.m.) rotating blade have largely left this catheter to be efficacious in only large peripheral arteries. Although a coronary version has been recently designed, it is cumbersome and cannot reach secondary coronary branches.
The smallest mechanical atherectomy catheter available has an outside diameter of 5 French. A catheter of this size requires an 11 French guiding catheter for insertion into an artery. The practical limitations on such a mechanical device are due to the difficulty in miniaturizing the mechanical parts and the inherent rigidity of a metal housing. The metal housing is necessitated by the presence of a rotating blade. Thus, the large metal housing associated with the catheter excludes the use of this device in small, tortuous vessels.
Laser atherectomy devices, on the other hand, can be as small as the laser optic component delivering the laser energy. In general, laser atherectomy involves directing a catheter, adapted to transmit laser energy, into a blood vessel and advancing the free end of the catheter within the blood vessel to the location of an occlusion or arteriosclerotic plaque. The catheter delivers laser energy to the location to vaporize the occlusion, thereby opening obstructed blood vessels.
The use of the intense and concentrated energy of a laser within a portion of the body, such as a blood vessel, presents the danger of damage to the surrounding tissue. In the case of a blood vessel, possible perforation of the blood vessel is a primary concern.
Atherectomy methods which utilize laser energy for treating vascular obstructions can be categorized as direct or indirect techniques. Direct ablation of atheromatous material usually produces prothrombotic thermal charring and surrounding acoustic or "blast" injury to the adjacent vessel wall The relatively long wavelengths associated with CO.sub.2, Nd:YAG, and argon lasers (10.6 u, 1.06 u, and 0.5 u respectively) contain minimal energy with which to cut through plaque material. Thus, such lasers must "hack away" at the obstruction leaving behind a ragged surface and producing a significant amount of thermal energy that can damage the surrounding vessel.
In comparison, the excimer laser has a shorter wavelength and a higher energy which enables it to vaporize tissue instantly without burning or causing acoustic injury. Excimer lasers utilize an inert gas and halogen gas medium and emit at a wavelength in the ultraviolet range. The power associated with the excimer laser (and rapidly pulsed, high energy lasers of longer wavelengths) is much more focused than the above lasers and permits ablation of the plaque before any heat is transmitted to the surroundings.
The direct use of lasers within arteries, however, is further hampered by the fact that lasers cannot distinguish plaque from the normal wall of the artery and, ultimately, perforate the arterial wall. Efforts to reduce the incidence of arterial perforation due to direct laser techniques are aimed at improving the sensitivity of plaque over normal wall (including spectral analysis, and tetracycline or hematoporhyrin staining), and at improving the delivery of laser energy along the vessel lumen (using guidewires, centralizing balloons, or angioscopic visualization of the laser fiber). None of these techniques provides a practical solution to safe ablation of plaque, sparing normal vessel wall, to date.
U.S. Pat. No. 4,207,874, issued June 17, 1980, for a "Laser Tunneling Device" is an example of a catheter having a bundle of optical fibers. The catheter is adapted to be advanced within a blood vessel to a point which is adjacent to an occlusion or calcified plaque for the application of the laser energy. Suction is applied to the laser to remove the debris resulting from the vaporization of the occlusion.
U.S. Pat. No. 4,240,431, issued Dec. 23, 1980, for "Laser Knife" discloses a device using laser energy for the incision or excision of an affected part of the body. In order to prevent the laser energy from causing undesirable cautery or piercing of normal tissue adjacent that which is to be treated, the laser energy is intercepted by a receiving surface, once the cutting procedure is completed.
U.S. Pat. No. 4,685,458, issued Aug. 11, 1987, for "Angioplasty Catheter and Method for Use Thereof" discloses a device having a pair of abutments disposed on the outer surface of the distal portion of a catheter. A fiber optic, either a bundle of glass fibers or a single fiber, has a distal end portion disposed within one abutment. The fiber optic is adapted to transmit laser energy from a source which is intercepted by the second abutment. A suction port disposed between the pair of abutments removes the debris from the site of the plaque at the inner surface of the blood vessel. The device has an inflatable bladder which causes the pair of abutments to contact the inner surface of the blood vessel. The device may be rotated about the central axis of the blood vessel. Removal of the plaque or occlusion is observed by fluoroscopy or taking a pressure gradient observation across the lesion.
The configurations of the direct laser atherectomy catheters described above do not provide for the positive containment of the resulting excised obstruction. Positive containment is a more reliable method of trapping all liberated obstructions and, thus, minimizes the potential for distal coronary embolization or loss of material into the systemic arterial circulation. It also avoids the possibility of suction-induced injury to the vessel wall.
Another drawback to the above laser atherectomy techniques is that the optical fibers are immobilized at one end of the obstruction. Thus, the delivered laser energy, which decreases in intensity over distance, merely chips away or disintegrates the obstruction, generating small particles. The small particles which easily elude capture may subsequently cause blockages in small blood vessels. The small particles which are captured are useless for diagnostic analysis. Retrieval of excised sections is an important diagnostic tool for determining the proximity of the excised section to the vessel wall and for distinguishing specimens of atheromatous plaque from thrombi specimens. The former function is useful to determine the extent of excision; the latter function is useful for prescribing post catheterization treatment.
Indirect laser techniques involve the controlled delivery of thermal energy to the diseased vessel. Examples of indirect thermal treatment include a catheter with a laser-heated metallic tip to "melt" plaque and a specially designed balloon catheter with light-diffusing fiber. The disadvantage of these treatment methods is that they merely reshape, rather than remove, plaque. Thus, these devices resemble balloon angioplasty more so than atherectomy, although the use of heat may leave behind a smoother surface than conventional angioplasty. See Sanborn, T.A., 55 J. Am. Coll. Cardiol. 934-38 (1985).
It is clear that a method is needed to enlarge arterial lumens that is more effective than angioplasty, more widely applicable than mechanical "cut-and-retrieve" systems, and safer than current direct and indirect laser techniques. A preferred treatment for vascular obstructions must minimize or correct local injury responsible for abrupt vessel reclosure and physically remove and/or leave behind a smooth luminal surface in an effort to reduce the incidence of thrombosis or restenosis. It is also clear, however, that while direct application of high energy lasers has the capacity to make surgically smooth excisions through atheromatous plaque, the risk of arterial perforation is great. Thus, an apparatus and method which prevent arterial perforation without detracting from the effectiveness of the direct application of laser energy is desired. Finally, a laser technique that collects excised obstructions suitable for diagnostic analysis and that controls the occurrence of debris entering the blood stream is needed.